This invention pertains to systems and methods for detecting bioelectrical activity. In particular, the invention relates to systems and methods for discriminating between cardiac depolarization and other sensed signals.
Cardiac rhythm management devices, such as pacemakers and implantable cardioverter/defibrillators, must accurately detect heart beats resulting from cardiac depolarization in order to perform their function. Such beat detection involves sensing a signal and then distinguishing between a signal due to cardiac depolarization and a noise signal due to either other cardiac electrical activity (e.g., abnormally conducted impulses, repolarization) or external noise. External noise sources that can generate sensed signals include electromagnetic fields in the environment and potentials produced by the patient""s muscular activity. A common method for reducing the risk of mistaking one of these noise signals for a cardiac depolarization is to use a sensing threshold designed to be above the average amplitude of the noise signal but below a depolarization signal. In conventional pacemakers with sensing channels for sensing one or more heart chambers, the sensing channels must also distinguish between ventricular and atrial depolarizations. One way of facilitating this is to render a sensing channel refractory following certain events. (The term xe2x80x9crefractoryxe2x80x9d means a no sensing condition when applied to a sensing channel, to be distinguished from the physiological refractory period of excitable tissue.) Sensing channels are rendered refractory both in order to prevent reentry into the system of an output pacing pulse (in which case the sensing amplifiers are blanked) and to prevent the misinterpretation of input data by the sensing of afterpotentials or by crosstalk between sensing channels.
Both of the above-mentioned methods for reducing the risk of oversensing, however, necessarily increase the risk of undersensing, i.e., that a depolarization event will fail to be detected. The present invention is directed toward an improved method for beat detection and discrimination that reduces the need for sensing thresholds and refractory periods.
The present invention is a system and method for detecting specific depolarization events such as occur in cardiac or other excitable tissue. It is particularly applicable to implantable cardiac devices such as pacemakers that function in response to detected heart beats, where the sensing channels of such devices are configured to sense either atrial or ventricular electrical activity in order to generate pacing pulses according to a programmed pacing mode. In accordance with the invention, cardiac electrical activity is sensed by a sensing channel to generate a sense signal. The sense signal is then processed in order to detect a specific cardiac depolarization event, such as a ventricular or atrial depolarization, by decomposing the signal into linearly independent components using orthogonal filters. A feature representing a statistical characteristic is then extracted from each signal component in synchronous fashion. The resulting feature set can then be compared with feature sets extracted from known signals (referred to as template signals) in order to identify the signal as a specific depolarization event.
In an exemplary embodiment, the sense signal is decomposed into multiple frequency components by a bandpass filter bank having passbands that correspond to the frequency components of a template signal that represents the specific cardiac depolarization event that is to be detected. In order to emphasize the high frequency components reflective of a change in signal energy, the frequency components of the sense signal are differentiated to extract a derivative signal therefrom. The sense signal frequency components are then amplitude demodulated to detect envelope signals representative of the signal energy changes in each of the passbands. The envelope signals are processed to determine if the signal energies of the passband frequency components of the sense signal increase in a phase-locked fashion. In a particular embodiment, the envelope signals are synchronously combined together by multiplying to generate a composite signal that is indicative of the detection of a depolarization event. In this manner, a cardiac depolarization event, such as an atrial or ventricular heart beat, can be reliably detected and discriminated from other signals.